In the conventional process for fabricating seamless tubing with multi-layered metals, coextrusion is generally applied. The extrusion billet is assembled with multiple layers of metallic cylinders one inside the other concentrically. The annular opening at each end of the assembly is sealed by vacuum welding. The billet assembly is preheated to a proper temperature and extruded, forming a metallurgical bond between adjacent layers. The bondline defects are occasionally detected at the as-extruded stage or after the tube is further reduced. The main cause for this problem is due to insufficient time at temperature to effect bonding and diffusion across the bondline during extrusion.
In the conventional process for preparation of two-layered zirconium-zirconium alloy billet, the outer zirconium alloy and inner machined zirconium component are assembled after cleaning and pickling. The annular opening at both ends of the assembly is sealed under vacuum by electron beam welding. The billet assembly is preheated to a temperature range of 600 degrees C. to 750 degrees C. and extruded. The bond between two metallic layers is effected by the extrusion process. Because of the short duration of extrusion process, bondline defects may occur at points between these metallic layers. Products for nuclear applications often require stringent inspection, such as ultrasonic techniques, bondline defects as small as a few thousandths of an inch in dimension, may still be detected. The present invention provides a process to improve the bonding between metallic layers of the coextrusion. It also provides a method to heal the bondline defects for the tube already extruded or further reduced from the billets, particularly those which were not first treated by the HIP process.
It is, therefore, an objective of the present invention to reduce or eliminate the occurrence of bondline defects in concentric mating surface for tubing having multiple metallic layers.
It is a further objective of the present invention to promote maximum bond strength with diffusion across the interface between the inner and outer metallic layers.
It is still another object of this invention to provide a process which is capable of healing bondline defects in extruded tubes.